DIGIDESIGN PRO TOOLS HD

Digidesign Pro Tools HD Digital Audio Workstation

Published in SOS May 2002
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Reviews : Computer Recording System

Photos: Mark Ewing
The new generation of 'high definition' Pro Tools systems promises superb sound quality and impressive DSP power.


Hugh Robjohns

Digidesign's Pro Tools has, in a decade or so, become one of the most widely used hard disk-based digital audio workstations -- or DAWs -- throughout the music and post-production industry. Although Pro Tools has evolved through several generations of software and hardware in that time, the basic system structure has remained constant. Powerful DSP cards are mounted in a host Mac computer, while external supporting hardware handles analogue and digital I/O requirements as well as enabling timecode synchronisation with other studio equipment.

The latest generation of Pro Tools, Pro Tools HD, maintains this structure but takes advantage of the continuing advances in DSP technology as well as better converters, higher sampling rates and improved clocking techniques. The Pro Tools software is essentially unchanged in appearance and features from the previous generation, Pro Tools 24 Mix, so the focus of this review will be the new system's hardware peripherals and overall technical structure.

The HD Structure

Although the Pro Tools HD operating software (version 5.3) will appear to be much the same as recent 24 Mix versions to the end user, the underlying DSP code is very different as the new HD hardware is vastly more powerful and capable than that of any previous generation. However, Digidesign appreciate that not everyone will require (or be able to afford) this new advanced level of sophistication, and so the current 24 Mix system will remain available as an alternative system for the foreseeable future, frozen at version 5.13 (or v5.2 with Rocket functionality). As usual, Digidesign are offering full upgrade paths from 24 Mix to HD, and recent purchasers of the 24 Mix system are apparently being offered very attractive step-up options.

Digidesign Pro Tools HD
pros
Sounds great!
TDM 2 affords greatly enhanced DSP capability.
Elevated sample rates.
Better clocking systems.
Retention of 'legacy' interfaces.
cons
New plug-ins required.
A high-end system with high-end pricing.
Not all hardware options fully functional or available yet.
summary
One of the world's leading DAWs has just got a whole lot better. The software is essentially the same as that of the current 24 Mix system, with a little polishing and extension here and there. However, the hardware is a radical departure from previous efforts with enormously enhanced capability, flexibility and performance.

Sadly, while the raw audio files remain compatible between the two systems, there is little compatibility between the internal or external hardware (although the Sample Cell card is still supported in the HD system, albeit without using the original TDM connections which are incompatible with the HD internal interfaces). The HD hardware can not be used with the 24 Mix DSP cards, and nor can 888 interfaces be used directly with the HD Core or Process DSP cards. However, Digidesign have thoughtfully provided Legacy ports on the 192 and 96 I/O interfaces (see below) to enable 888 interfaces to be linked through the new hardware. This provides a cost-effective way of expanding the I/O capability of the complete HD system by reusing existing hardware.

The new HD Core and Process DSP cards look very similar to each other. Each is loaded with a stack of nine 100MHz DSP chips and apparently provides 25 percent more processing power than a 24 Mix card (although these used only six 80MHz DSPs). One very obvious benefit of that increase in DSP power is the expansion of the EQ facilities to four bands, but an equally obvious drawback is that 24 Mix TDM plug-ins are incompatible with the new cards because of changes to the DSP code. However, all is not quite doom and gloom on the plug-in front: the DigiRack collection of standard plug-ins is supplied as standard with the software, and the majority of plug-in suppliers have either rewritten or are in the process of rewriting their tools for the HD system. At the time of writing, third-party manufacturers who already have HD-compatible plug-ins include Access, Aphex, Bomb Factory, Drawmer, DSP, DUY, Focusrite, Line 6, Waves and Wave Mechanics. Already well down the line, so to speak, are Antares, Dolby, Metric Halo, Sony and Waldorf, to name but a few.

Another vitally important difference between the 24 Mix DSP structure and that of the new HD system is a totally different buss architecture called (rather unimaginatively) TDM 2. A time slot is required on the TDM buss for every audio sample passed to or from an I/O port, and for audio data to pass between DSPs. The previous TDM architecture, as employed in the 24 Mix system and its forebears, provided 256 DSP buss time slots for the entire system, and as a consequence, bottlenecks often formed in the TDM bussing when very complex Pro Tools mixing desks and track configurations were employed. This 256-time-slot restriction was fine when DSPs were rather less capable than they currently are, but formed a significant limitation to the flexibility and capability of the more powerful DSPs available today.

The new TDM 2 structure has been engineered to provide 512 time-slots between each DSP at the basic sample rates -- there is a corresponding reduction at higher rates. The vast increase in capacity thus afforded provides a huge increase in processing power, enabling the true DSP capacity of the new Core and Process DSP cards to be realised. Digidesign suggested to me that it would be impossible to create data bottlenecks on a TDM 2 system... so consider that a challenge!

Standard Configurations

There are three standard HD system configurations, each based around a single Core card which can handle up to 32 channels of I/O via the 192 or 96 I/O units and provide up to 96 simultaneous audio tracks ('voices') at the base sample rates. The HD1 system contains a single Core card, whereas the HD2 adds a single Process card to this and the HD3 system adds two Process cards. Each additional Process card is linked via an internal Digilink cable instead of the old TDM bussing arrangement, and increases the possible I/O by a further 32 channels. So the HD3 system, for example, can accommodate 96 channels of I/O and 128 simultaneous audio tracks or voices at 44.1 or 48kHz sample rates.


A welcome feature of the new HD hardware is that it is entirely configurable from within the Pro Tools software.
If operating at 88.2 or 96kHz sample rates the maximum number of voices (audio tracks) falls to 48, and to just 24 tracks at 176.4 or 192kHz. When configuring the I/O interfaces from the Pro Tools menus, the structure is pretty much the same as with the 24 Mix system, except for the inclusion of rather natty graphic icons to make it more obvious which interfaces are being controlled.

Up to six Process cards can be linked to a Core card if required -- the amount of I/O cannot exceed 96 channels, but the amount of mixing and plug-in processing power will continue to increase with additional cards. The exception is that at 192kHz sample rates, the maximum number of cards the system can employ is four (one Core and three Process) because of interface timing issues. Given that the system is also limited to 24 audio tracks at this sample rate, I don't think anyone is likely to find this restriction of I/O numbers a significant problem.

When working at elevated sample rates Digidesign recommend using four SCSI drives simultaneously (FireWire drives are currently being tested for their suitability and reliability). The audio data format defaults to WAV files in v5.3, although SDII and AIFF file formats are still available as user-selected options if required.

Sync I/O

Digidesign have designed a total of five hardware interfaces for the Pro Tools HD system, of which three are available now and the other two should be launched soon. The new units share a distinctive new look with very modern curved silver front panels and blue sculpted rack ears -- a major improvement on the dull black rectangular panels of the familiar old triple-8s.

The three units I tested (and which are shown in the photo on the first page of this article) are the two audio I/O systems, the 192 and the 96, and the Sync I/O. The last is not really new -- it is more or less a reboxing of the existing Universal Slave Driver (USD) synchronisation unit, although Digidesign claim to have improved the clocking and jitter specs, and have obviously added the high sampling rate functionality as well. The rear panel is almost identical to that of its predecessor, the only obvious difference being the replacement of the Superclock BNCs with a pair of Loopsync BNCs (see box), an important clocking element of the new HD system. With the restyling the front panel looks different, but much the same facilities and controls are to be found there.


The Sync I/O supports all major timecode and clock formats, including Digidesign's new Loopsync clocking technology.
The function of the Sync I/O is to act as the master synchronisation source for a complete system when used in conjunction with video machines, or when working with timecode-slaved audio transports of various kinds. To that end the Sync I/O accepts and can slave to all standard timecodes in either LTC (normal 'audio' timecode) or VITC (video picture timecode) formats, and can accommodate the complex pull-up and pull-down requirements of the film/video industries. It can slave to bi-phase signals from film bays as well as all the usual word clocks, AES and video references, and can also generate timecode in LTC and VITC formats plus MIDI timecode (MTC), with associated word clock, Superclock and AES references too. The timecode display can be 'burnt in' to a video signal if required. Two nine-pin ports allow the serial control of two transports or synchronisation systems via Digidesign's optional Machine Control. Communication and control between the Sync I/O and HD Core card is via a serial cable.

Although the AVoption and AVoption XL facilities have not been implemented in the first release of Pro Tools HD, they will be incorporated in subsequent releases and Sync I/O unit will then enable integration with Avid picture-editing systems. The unit can be configured and controlled from its own front panel or remotely, either from a dedicated Setup application or through the Pro Tools software itself. It defaults to being controlled through Pro Tools.

During my review the Sync I/O was fully functional but awaiting a firmware update to enable full integration with the rest of the HD system. In particular, it was not able to operate within the Loopsync clocking structure, although this did not stop it from fulfilling its principal role -- it merely made the remote reorganisation of clocking within a combination of hardware a little less flexible than it will eventually become. I found that the Sync I/O synchronised the connected Pro Tools HD system to incoming timecode very quickly, maintaining almost sample-accurate lock even with 192kHz sample rates.

  Other Hardware Interfaces  
  The Digidesign publicity machine has already published quite a lot of information about the two yet-to-be-released hardware interfaces in the HD system. The first is a multiport MIDI interface with 10 MIDI inputs and outputs (ports 9 and 10 are reproduced on the front panel for ease of access). It is designed so that input 1 is automatically routed to all ten outputs when the USB-connected Pro Tools computer is turned off, and can support up to four further MIDI I/O peripherals to provide up to 40 MIDI ports! The unit is designed to be powered via the USB connection, although a co-axial power socket is also included.

However, perhaps its most interesting feature is Digidesign's Time Stamping facility, which should ensure precise (better than 1ms) timing accuracy with the audio recorded in Pro Tools, and freedom from jitter. It should be noted that the current release of Pro Tools HD does not yet incorporate this new time-stamping software module... but since the MIDI I/O unit is not yet shipping either, this is of little consequence at the moment!

The final HD hardware product is the Pre, an eight-channel remote-controlled microphone preamplifier developed in conjunction with Focusrite. Each channel has a high-quality mic input stage using an XLR connector, and a separate line/instrument input on a TRS jack (the first two channels have additional, paralleled line/DI inputs on the front panel too), as well as independent insert send and return TRS sockets. There is also an internal oscillator for calibration and line-up purposes, with an independent output, and a trio of MIDI ports enables the machine's parameters to be controlled remotely.

Each channel is provided with up to 66dB of gain, switchable phantom power, high-pass filter, polarity reversal, insert, input impedance selection (1.5k(omega), 15k(omega) or 1.5M(omega)) and a switchable soft-clip limiter circuit. Every parameter can be controlled remotely. The eight discrete channel outputs are dispatched via a D-Sub connector to interface with a 192 or 96 I/O, although a suitable breakout cable would allow the Pre to be used with other, non-Pro Tools equipment just as effectively.

The Pre can be used as a stand-alone multi-channel mic preamp, and controlled either from its front panel, or via standard MIDI commands -- this might allow the preamp to be sited in the studio closer to the microphones, for example. Up to eight Pre units can be chained together for control via MIDI, either from Pro Tools, one of the Digidesign control surfaces, or any other standard MIDI control surface. Also, being fully MIDI controlled, it is possible to store and recall instantly the unit's complete configuration for a particular session.

Both the Pre and the MIDI I/O are slated for launch around the time you read this.

 

New 192 I/O Interface

The flagship I/O interface for the |HD systems is the new 192 I/O, which Digidesign claim to be the best-sounding audio interface they have ever offered -- and I don't suppose anyone would wish to argue that point. This is a brand-new design, not an evolution from the triple-8s, and it offers some interesting new facilities.

Fitted with a combination of eight-channel interfaces, this unit is capable of passing up to 16 channels to and from the HD Core or Process card to which it is attached. The 16 channels can be derived from any of the physical I/O available, through the software configuration pages in Pro Tools. The 192 I/O also features modular rear-panel sections to allow a degree of I/O format customisation, of which more in a moment.


The 192 I/O boasts an impressive array of analogue and digital connectivity, and can be expanded through the addition of further analogue inputs or outputs, or additional digital I/O.
The sleek-looking front panel carries a wealth of LEDs but no operational controls whatsoever (apart from a power button, that is -- and even that is ringed with green LEDs!). There are six LEDs to show the current sample rate (44.1, 48, 88.1, 96, 174.4 and 192kHz), and a further five to indicate the clock synchronisation mode. Internal (Int), external (Ext) and digital input (Dig) are probably all self-explanatory, but the two new ones are Loop and Loop Master, and refer to Digidesign's new flexible clocking system which enables complex systems to be configured remotely. The Superclocks And Loopsyncs box within this review explains this intriguing system in more detail.

The right-hand side of the panel carries four eight-channel arrays of metering LEDs, configured to show the signals present on each of the possible 16 input and output channels. Each bar-graph consists of just four LEDs but they cover a wide dynamic range, with the bottom two green LEDs signifying signal levels of -42 and -18dBfs, while a yellow LED illuminates at -6dBfs and a red at zero.

The rear panel is divided into three main sections, the two on the left being split horizontally to carry up to two optional interface cards each. By default, the 192 is fitted with eight-channel 24-bit analogue input and output cards, and an eight-channel digital I/O card which carries separate AES-EBU, TDIF and ADAT optical ports. The digital inputs are also equipped with hardware sample-rate converters allowing non-synchronous or incompatible sample-rate audio to be imported without fuss. The specs suggest the analogue inputs benefit from a signal-noise ratio of 120dB (A-weighted), with distortion below 0.00035 percent, and the analogue outputs offer only a very slightly inferior performance, with a 117dB (A-weighted) dynamic range and 0.0007 percent distortion.

The I/O is configurable to meet each user's specific needs: if more analogue inputs are required, an extra ADxpand card can be installed, while another digital card can be added if more digital I/O is necessary. All connections on the analogue and digital cards (with the exception of the ADAT opticals) are via D-sub connectors.

  Prices  
  HD Core systems:
* HD1: £7037.
* HD2: £8788.
* HD3: £10550.
* Individual Process cards: £3512.
* 192 I/O: £3512.
* 96 I/O: £1761.
* Sync I/O: £1844.
* ADxpand interface for 192 I/O: £1139.
* DAxpand interface for 192 I/O: £1056.
* Digital expansion card for 192 I/O: £880.
* MIDI I/O: £528.
* Pre: £2196.

Prices include VAT.

 
The analogue input has connectors for both +4dBu and -10dBV nominal level inputs with A and B preset level trimmers. A software-activated soft-clip limiter is also provided. This is very similar to Apogee's well-known Soft Limit function, and starts to compress the audio signal above about -4dBfs to avoid transient overloads, generating a kind of tape saturation effect instead. The analogue outputs have single connectors, but A and B output preset level trimmers again.

The right-hand side of the rear panel carries a variety of permanent interfaces including stereo AES-EBU and S/PDIF (co-axial) I/Os, another eight-channel ADAT optical I/O (which can be reconfigured as a stereo TOSlink interface), plus Loopsync in and out, word clock in and out, and the obligatory IEC mains inlet. Below these connectors is a row of four multi-pin ports, the first two being new-style Digilink connectors, used to interface with the Pro Tools HD DSP computer cards and other HD interfaces. These are labelled as Primary and Expansion port: the first links with an HD Core or Process card in the host computer, and the second allows a second HD I/O unit to be connected to the system through the first. This facility allows the full 32-channel capacity of each Process card to be utilised in a very convenient manner.

The next socket is labelled 'Legacy Peripheral' and allows connection of the previous generation of 888, 1622 or ADAT Bridge interfaces via the 192 I/O -- handy for those upgrading from earlier systems and wishing to maximise the working life of their existing hardware. Clearly, using a legacy I/O unit will limit the sample-rate options to the lower rates, and an older interface cannot be used if the Expansion port is already in use -- this is an either/or facility. A fourth socket, this time a nine-pin D-Sub, is labelled Accessory and is intended only for testing and maintenance purposes at this time.

One of the most practical advances of the HD software is the automatic recognition and configuration of connected HD hardware. Thus there is no longer a need to configure the system manually to declare which interface is connected to which Core or Process card -- a blessing indeed! However, should you want to check which interface is connected to which Process card, there is an Identify function within the Pro Tools setup menus which flashes all the front-panel LEDs of a selected unit.

96 I/O Interface

The 96 I/O unit is similar to, but less highly specified and configurable than its much more expensive sibling, with no options on the interfacing formats at all -- you have to have only what is provided! In effect, this is the replacement for the 888/24 employed as the primary interface for the 24 Mix system, but offers far better performance at a substantially lower price. It is not, however, compatible with the 24 Mix hardware, so these attractive qualities are of no practical use to owners of older systems.


The 96 I/O is a more-or-less direct replacement for the older 888/24 interface, and although it is not expandable (despite the presence of blanking panels!), offers a decent quotient of analogue and digital I/O as standard. Older 888, 1622 and ADAT Bridge interfaces can be used with both the 96 and the 192 thanks to the Legacy port.
The front panel is similar to that of the 192 I/O, but carries only two eight-way arrays of LEDs for the 16 possible channels. However, these can be switched remotely (from the Pro Tools software) between input and output metering, a pair of adjacent LEDs indicating the current mode. Synchronisation and sample rate LEDs are provided as found on the 192 I/O, although there are only four sample rate options here: 44.1, 48, 88.2 and 96kHz.

The rear panel initially looks similar too, but it is not apparently possible to plug in alternative interface cards. The unit is supplied with just two eight-channel balanced analogue interfaces, one for inputs and the other outputs -- both using TRS quarter-inch jack sockets -- and without level trims. The section to the right of the rear panel is identical to that on the 192 I/O, with the same collection of digital and clock interfaces: stereo AES-EBU and S/PDIF, eight-channel ADAT optical ports reconfigurable as stereo TOSlink, and the same word clock and Loopsync facilities. The HD Primary and Expansion ports are also present along with the Legacy port and Accessory connector.

The 96 I/O's tech specs are not quite as impressive as those for the 192 I/O, but are not far short, with 115dB (A-weighted) dynamic range and 0.0007 percent distortion in the A-D converters, for example.

  Superclocks And Loopsyncs  
  Another significant change from previous Pro Tools systems is the passing of the Superclock. In my experience, the x256 Fs Superclock system was often more trouble than it was worth, with few users understanding the technical implications of running clock signals around an installation at this kind of rate.

With the move up to 192kHz sample rates, Digidesign have realised that trying to maintain a Superclock at the appropriate frequency (about 50MHz) is just not practical. Although all of the new I/O interfaces can still accept and generate Superclock signals (for compatibility with 888 units and the like), it is no longer an essential part of the system.

Digidesign have paid a lot more attention to the whole issue of clocking and clock accuracy with their new HD hardware, particularly since many high-end users of 24 Mix systems chose to use Apogee and Prism interfaces rather than Digidesign's triple-8s because of their allegedly greater clock stability (and better audio resolution).

Part of this major overhaul of clocking systems is the introduction of 'Loopsync'. For anyone familiar with the nuts and bolts of clocking digital equipment this is, on first sight, a fairly scary proposition. Each item of HD hardware connected in the system is wired into a continuous loop of word clock signals, the Loopsync output from one unit being connected with short BNC leads to the Loopsync input of the next. Its output is then connected to the input of the next unit, and so on until the last unit in the chain has its output connected back to the input of the first machine. Try this with normal word clock inputs and outputs and the mother of all clock howlrounds is likely to ensue, with a corresponding amount of misery!

However, the HD hardware is sufficiently intelligent that the 'most senior' unit in the system recognises that fact and automatically assumes responsibility as clock master. This would normally be the first 192 I/O (as the unit with the highest sample-rate capability), but could equally well be the Sync I/O in suitably equipped systems. The Loop Master LED on the front panel of the master machine illuminates to proclaim its assumed role as clock master, and all other units put their Loopsync lights on, to show that they are being clocked via the Loopsync system. As the clocking signal (at the declared sample rate -- none of that x256 nonsense!) is passed from one unit to the next it is cleaned up and de-jittered to try to maintain the best possible clock accuracy throughout the system.

The advantage of this arrangement is that any piece of equipment connected into the Loopsync ring can be can be designated as the Loop Master from the Pro Tools software, without having to delve around the back replugging lots of spaghetti -- a feature many users will be very pleased about.

Of course, there can only ever be one Loop Master in the system at any time... but if you have attended any of the Pro Tools HD demonstrations at trade shows or as part of the company's nationwide tour you may well have spotted two Loop Masters! In systems which combine both the 192 I/O and the Sync I/O, both units will probably have been claiming Loop Master status. This is because the current firmware in the Sync I/O does not support the Loopsync structure. This is an issue for current HD users, but should be resolved by the end of the summer.

 

Listening Tests

Clearly, the Pro Tools HD system offers a lot over the previous 24 Mix platform, both in terms of its elevated sample rate capabilities and sheer DSP power. It is also clear that Digidesign have tried very hard to resolve the clocking issues of the old 888/24 and 888/20 interfaces to bring the audio resolution up to the standards obtainable from other hardware and software digital recording systems. I thought it would be worth running a direct comparison between the HD I/Os and an old triple-8 interface to see just what kind of improvement had been made. (This experiment was not held in an ISO standard listening room with double-blind laboratory conditions, but in a small workshop at Digidesign's premises on the Pinewood film lot, so I can't vouch for the scientific accuracy of the following observations -- these are subjective opinions!)

The setup I tested was an HD3 system with 192 and 96 I/O interfaces, with an 888/24 connected to the Legacy port of the 192. The main stereo outputs of the Pro Tools system were routed to both the 192 and 888, their analogue outputs being fed through the monitoring section of a Pro Control desk and carefully matched in level. The monitoring was courtesy of Genelec 1031s.

Running a variety of material from the Pro Tools at 48kHz sample rates, we switched between the two and listened for any differences. It wasn't hard to spot that the HD hardware created more stable and coherent images of the acoustic instruments, and conveyed a greater sense of depth and spaciousness. I also believe the HD hardware sounded slightly smoother and less coloured, although this was a more subtle difference.

We then ran the same material at its original sample rate of 96kHz (it had been rate-converted for the previous demonstration to accommodate the 888's sample-rate limitations). This proved to be quite a good demonstration of just what it is that 96kHz recording brings to the party. The acoustic instruments (violins, violas and guitars mainly) became significantly more 'real'. The system conveyed much more information about the wood of the instruments, their size, movement and relative spatial positioning -- it was as if an acoustic veil had been removed. I know this is starting to read like a hi-fi magazine, but the difference really was that obvious. Whether Joe Bloggs would notice the improvements in your recordings on the £99 ghettoblaster he bought in Currycomets at Christmas is another matter entirely...

Overall I was very impressed with the Pro Tools HD system. The outboard hardware appears to exhibit considerable technical and sonic advances over the previous black-fronted boxes, and the DSP power of the Core and Process cards is simply mind-blowing. The user interface remains entirely familiar so no one need fear the upgrade in operational or sonic terms. However, the damage to the bank account will be quite substantial, and it will be a very long time before HD becomes as common as earlier Pro Tools incarnations.

High-end studios will probably upgrade, especially those recording large orchestral sessions -- the Abbey Roads of the business -- partly because the high sample rates really do benefit this kind of material. And plenty of other users will move onto the HD platform simply because it is newer, technically better, and more expensive! Also, those who use complex virtual mixers and numerous plug-ins may appreciate the benefits of the TDM 2 architecture and its 'unbreakable' DSP resources. However, I feel that a majority of average 'pop music' users, as well as those using the system for film, video and broadcast post-production applications will probably see little real benefit, particularly if they are already using Apogee or Prism interfaces.

With even the most basic HD 1 and 96 I/O system costing nearly £9,000, this is an expensive DAW, albeit one of the best. At the end of the day, though, there are lots of technical innovations and step-improvements in capabilities and performance which mark this out as a class-leading product.

 information
See 'Prices' box.
Digidesign UK
+44 (0)1753 653322.
+44 (0)1753 654999.
Click here to email
www.digidesign.com

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